Why is my rigidbody broken?
For this kind of shape (the zero) using convex hull seems to work better than using mesh.
From the manual page on collisions:

Convex Hull
A mesh-like surface encompassing (e.g. shrink-wrapped over) all
vertices (best results with fewer vertices). A convex approximation of
the object, which has good performance and stability.


Mesh
Mesh consisting of triangles only, allowing for more detailed interactions than convex hulls. Allows simulating concave objects, but
is rather slow and unstable.

In your case, using convex hull stabilizes things much more.

Here is the difference between mesh and convex hull in your case:
Mesh:

Convex Hull: